1. Field of the Invention
The present invention is directed to a technique for measuring color cathode ray tube (CRT) luminance or output for each primary gun of or color produced by the CRT and, more particularly, to a technique that biases the measured signal into a measurement range of a sensor by turning on, partially or fully, one or more electron beam guns for colors other than the color being measured, thereby allowing measurements above a noise level to be made for the remaining colors.
2. Description of the Related Art
To successfully calibrate a color cathode ray tube (CRT) or monitor, it is necessary to measure the light intensity output of the individual red, green, and blue (RGB) primary guns at varying voltage level or digital luminance control values. This can be done by displaying varying intensity primary color patches on the CRT and using an optical power meter or some other light measuring device to measure the light intensity in foot lamberts (fL.) or some other light intensity measurement unit. Typically, the photopically measured light intensity emitted by the blue gun alone at 100% intensity is 1.9 fL. which is very low compared to the green gun which at 100% intensity produces 15.51 fL. Inexpensive light intensity measurement sensors, such as the Kodak part number 237092 available from Eastman Kodak Co. and used in Kodak cameras, such as the model S500, exhibit noise at low light levels and are therefore not suitable for blue gun measurement. For example, the typical blue gun outputs 0.09 fL at 25% full intensity and in most of the operating range of the blue gun, the light intensity produced is less than the 1.5 fL. noise level mentioned above. To accurately measure the light output of the blue gun, an expensive, more accurate measuring device must be used. Further, at such low light levels, the measurements are less accurate due to ambient light infiltration into the glass of the CRT.
What is needed is a measurement method and apparatus which allows accurate intensity measurements with a low cost sensor such as that mentioned above.